Wordpress won't touch it's own inventory.
"“it seemed like Google’s AdSense was the state-of-the-art, which was sad. You put a lot of time and effort into your blog, and you deserve better than AdSense,” he writes."
If you have a garbage wordpress hosted blog I guarantee you don't put time and effort into building a brand. Your inventory is the scum of the internet, that's why advertisers pay 0.001c per impression. And that is why Federated Media and WordAds also doesn't want your custom.